It has long been known to use micromachined silicon mass air flow sensors (MAFS) in applications including, for example, motor vehicle engine air intake sensors. Such sensors operate as a hot film anemometer sensor. Hot film anemometry employs a wire resistor or a thin film resistor deposited on a substrate, such as quartz, glass or silicon. Typically, associated sensor circuitry, usually IC circuitry, maintains the wire or thin film resistor at a certain temperature (or temperature differential) above the temperature of the fluid flow being measured. Any change in the flow correspondingly alters the cooling effect on the heated resistor. The associated sensor circuitry senses such change and generates a corresponding signal.
An important feature of a MAFS based on the hot film anemometry principle is good thermal isolation of the heated element from its supporting structure. In the MAFS device disclosed in U.S. Pat. No. 4,594,889 to McCarthy a monolithic silicon air flow sensor is disclosed wherein a silicon chip is micromachined using commercially known techniques to produce an air flow opening through the chip, the opening being divided by one or more silicon beams unitary with the silicon chip substrate. A thin film resistor is deposited on the silicon beams using commercially known metalization techniques. Relatively good thermal isolation is achieved by making the beam cross-section small compared to its length, for example, 50 micron by 50 micron cross-sectional area for a beam bridging a 5 mm air flow opening. Increasing the cross-sectional area of the silicon beams would increase the durability of the sensor. Smaller beams are more easily damaged by dust and other particulate matter entrained in the air flow being measured.
Larger beam cross-sections, however, while more durable, cause a larger portion of the heat dissipated on the beam to be lost to the silicon substrate. That is, heat flows from the end of the beam into the silicon substrate at the perimeter of the air flow opening. Thus, larger beam sizes would require more power to operate and the zero air flow offset factor is correspondingly large. Thus, larger cross-section beams have a disadvantageously reduced signal-to-noise ratio, making accurate air flow sensing more difficult. In addition, the temperature increase of the substrate resulting from use of a larger, more durable silicon beam can introduce an error in the measurement of the heat transferred to the air flow. Also, temperature compensation of the sensor is made more difficult, because the coefficient of thermal conductivity of silicon decreases with increasing temperature while the silicon-to-air heat transfer coefficient increases.
Thus, there is a need for a mass air flow sensor which has good durability and also good accuracy and reliability. It is an object of the present invention to provide a sensor having both good durability and good accuracy. Additional objects and advantages of various embodiments of the invention will be apparent from the following disclosure and discussion thereof.